Steam generating unit

ABSTRACT

This invention relates to a stream generating unit and, more particularly, to apparatus for generating steam in which the heat from the products of combustion is used very efficiently in a compact, prefabricated construction arrangement.

United States Patent Inventor Appl. No.

Filed Patented Assignee Ramsey Sheikh Westboro, Mass.

Apr. 26, 1968 Feb. 2, 1 97 l Riley Stoker Corporation Worcester, Mass.

a corporation of Massachusetts STEAM GENERATING UNIT 17 Claims, 5Drawing Figs.

US. Cl

Int. Cl F22!) 21/02 Field of Search [56] References Cited UNITED STATESPATENTS 3,182,640 5/1965 Miller 3,283,801 11/1966 Blodgett et al....2,825,312 3/1958 Kolling 1,859,858 5/1932 Wright 3,266,467 8/1966Winship 2,013,565 9/ 1 935 Lundgren 3,003,482 10/1961 Hamilton et al.3,272,186 9/1966 Lorenzini 3,227,870 1 H1966 Reale 3,315,647 4/1967Triggs Primary Examinerl(enneth W. Sprague Attorney-Ward, McElhannon,Brooks & Fitzpatrick ABSTRACT: This invention relates to a steamgenerating unit and, more particularly, to apparatus for generatingsteam in which the heat from the products of combustion is used veryefiiciently in a compact, prefabricated construction arrangement.

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STEAM GENERATING UNIT BACKGROUND OF THE INVENTION It has becomefashionable in the boiler industry for the past several years to statewith considerable pride that boilermakers have been continuallypessimistic in predicting the maximum practical size of package boilers.For instance, the largest package boiler fifteen years ago was for anevaporation of approximately 40,000 lbs/hr. Today, just a few yearslater, the maximum size is over 200,000 lbs/hr., or five times as large.It may be assumed that this expansion will continue and that in tenyears or so the boiler industry will be shipping boilers of 400,000lbs/hr. or larger in the form of premanufactured package boilers. Theexpression package boiler. when it relates to the larger sizes, nolonger describes a boiler which is lifted from a railcar, set on itsfoundation, and piped to steam, fuel and feedwater connections. Instead,the completed installation in many cases consists of several boilerassemblies, ducts, heat recovery equipment, fans, drives, etc. By andlarge, the common denominator for these units is the fact that the fieldlabor costs have been kept to a minimum. Usually, the net result is thatdelivered and erected first cost of a package boiler is substantiallybelow that of a field-erected unit of the same evaporation. This is thefundamental reason why there is a wide market for this type of boiler. Acomprehensive analysis of relative costs of package versus fielderectedboilers was recently made and it was concluded that cost comparisons aredifficult to make because of the fact that they are not truly comparabletypes of boilers. In addition, the variation in the cost of packageboilers themselves can be as much as 50 percent depending on whether theunit is conser' vatively rated or whether it is a marginal unit pushedto extremes of design. As the size of package boilers increases thispercentage spread becomes a significant dollar factor in the overallcost of an installation and should be examined very carefully. Inaddition to the above matters, a new factor has entered the situationwhich may affect the economics of package boilers in a very significantway. It is now almost universal practice to design field-erected boilersfrom the smallest industrial to the largest central-station size aroundthe welded wall-type of construction. This relatively new factor has asignificant bearing on the shop versus field costs of all boilers. It isinteresting to note that, generally speaking, package boilers with thistype of construction are conspicuous by their absence from the market.This is because this type of construction has been very difficult toincorporate into this type of boiler. The attempt to extend the knownpackage boiler designs into the larger sizes results in the difficultythat package boilers in general are designed with certain compromiseswith sound design practice. As these boilers become larger, however, theavailability of the boiler becomes much more important, since itrepresents a larger capital investment and more processes are dependentupon its generation of steam. These and other difficulties experiencedwith the prior art devices have been obviated in a novel manner by thepresent invention.

It is, therefore, an outstanding object of the invention to provide asteam generating unit of the package type in which the heat efficiencyis high.

Another object of this invention is the provision of a steam generatingunit in which the major elements are capable of construction in amanufacturing plant and shipped to the field for final assembly.

A further object of the present invention is the provision of a steamgenerating unit of the package type having high availability.

It is another object of the instant invention to provide a package steamgenerating unit having high steam generating capacity without attendantgas flow problems.

A still further object of the invention is the provision of a steamgenerating unit in which the fuel burning and gas flow arrangement issuch to promote excellent mixing of gases and absence of temperaturestratification.

With these and other objects in view, as will be apparent to thoseskilled in the art, the invention resides in the combination of partsset forth in the specification and covered by the claims appendedhereto.

SUMMARY OF THE INVENTION In general, the present invention consists of asteam generating unit having a vertically-elongated furnace section ofwelded wall construction, having a boiler section ofvertically-elongated form mounted parallel to and adjacent the furnacesection, having feeder tubes connecting the two sections at their lowerends, and having steam release tubes joining the two sections at theirupper ends.

More specifically, the furnace section is polygonal in cross section andburners are located on two of the sides which are adjacent to andangularly arranged relative to a rear wall. The furnace section isprovided with a bottom consisting of a flat central portion joined toupwardly-inclined portions on the sides. The header extends transverselyacross the bottom with horizontal and inclined portions lying in thecentral and inclined portions. respectively. In addition, the furnacesection is provided with a roof consisting of a horizontal centralportion joined to a downwardly-inclined portion on each side and aheader extends transversely across the roof with horizontal and inclinedportions lying in the central and inclined portions of the roof.

BRIEF DESCRIPTION OF THE DRAWINGS The character of the invention,however, may be best understood by reference to one of its structuralforms, as illustrated by the accompanying drawings, in which:

FIG. 1 is a perspective view of a steam generating unit embodying theprinciples of the present invention,

FIG. 2 is a longitudinal sectional view of the unit,

FIG. 3 is a sectional view of the unit taken on the line III-III of FIG.2,

FIG. 4 is a horizontal sectional view of the unit taken on the linelV-IV of FIG. 2, and

FIG. 5 is a sectional view of a portion of a wall.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. I,wherein are best shown the general features of the invention, the steamgenerating unit, indicated generally by the reference numeral 10, isshown as consisting of a furnace section 11 and a boiler section 12. Thefurnace section 11 is formed of welded wall construction of the typeshown in section in FIG. 5 and is of vertically-elongated form. Theboiler section 12 is also of vertically-elongated form and is mountedparallel to and adjacent to the furnace section. Feeder tubes 13 connectthe two sections at their lower ends and steam release tubes 14 join thetwo sections at their upper ends. Each section has a cross-sectionaldimension which permits transportation on a railroad flat car withadequate overhead and side clearance. The cross-sectional shape of thefurnace section 1 l is octagonal and burners l5 and 16 are located ontwo of the sides which are adjacent to and angularly-arranged relativeto the rear wall 17.

The furnace section 11 is provided with a bottom 18 which consists of aflat central portion 19 joined to upwardlyinclined portions 21 and 22 onthe sides. The header 23 extends across the bottom 18 with horizontaland inclined portions lying in the central portion 19 and the inclinedportions 21 and 22, respectively.

The feeder tubes 13 extend rearwardly from the header 23 and are weldedto corresponding feeder tubes extending forwardly from the boilersection 12. The furnace section 11 is provided with a roof 24 consistingof a horizontal central portion 25 joined to downwardly-inclinedportions 26 and 27 on the sides. A header 28 extends transversely acrossthe roof 24 with horizontal and inclined portions lying in thecentralportion 25 and the inclined portions 26 and 27, respectively.

The steam release tubes 14 extend rearwardly from the header 28 and arewelded to corresponding steam release tubes extending forwardly from theboiler section. The rear wall 17 of the furnace section 11 and a frontwall 29 of the boiler section 12 are located in juxtaposition. The frontwall 29 of the boiler section has two side panels 31 and 32 (see FIG. 4)which lie in a transverse plane and define a gap between them. The rearwall of the furnace section 11 has a central panel 33 which lies in thesaid gap and also has the two forwardly-inclined panels 34 and 35 whichlie at a substantial horizontal angle to the said side panels of thefront wall of the boiler section.

Each side panel 31 and 32 of the front wall 29 of the boiler section 12and its corresponding forwardly-inclined panel 34 and 35, respectively,of the rear wall of the furnace section form a space of triangularhorizontal cross-sectional shape in which lie burner boxes associatedwith the burners and 16. As a practical matter, a plurality ofvertically-spaced burners are mounted in each burner box and extendthrough the said forwardly-inclined panels 34 and 35, as is evident inFIG. 4. Each burner box is provided with a baffle 50 and dampers 51 topermit delivery of air selectively to an individual burner.

The aforementioned gap in the front wall of the boiler section isprovided with widely-spaced tubes 36 which extend from the upper drum 37to the lower drum 38 and these tubes are not joined in theirintermediate portions. The central panel 33 of the rear wall [7 of thefurnace portion 11 has some of its tubes bent out of the plane of thepanel in the upper portion to form a gas exit opening. The tube spacingand location in longitudinal vertical planes is the same as the spacingand location of the tubes in the widely-spaced tubes 36 in theaforementioned gap.

The boiler section 12 is provided with an upper drum 37 and a lower drum38 which are joined in the usual manner by a mass of downcomer tubes 39.The drums are joined by a front, rear, and two sidewalls formed ofclosely-spaced tubes which, in turn, are joined by welded strips. Abaffle 41 extends downwardly from the upper drum 37 to divide the spaceformed by the said walls into a forward pass in which a superheater 42is located and a rearward pass in which the said mass of downcomer tubes39 is located.

The furnace section 11 and the boiler section 12 are independentlymanufactured at a location remote from the construction site. As hasbeen stated above, the boiler section is provided withforwardly-directed feeder tubes at the lower end and steam release tubesat the upper end, while the furnace section is provided withrearwardly-directed feeder tubes at the lower end and steam releasetubes at the upper end. The sections are independently transported tothe construction site where they are mounted in parallel juxtapositionand the feeder and steam release tubes are welded together.

The advantages of the above construction will be apparent from a closestudy. For one thing, a safe margin of water circulation in the boilerhas been assured by providing a high-head boiler which permits thehandling of high heat absorption rates that are inherent inshop-assembled boilers. Since both sections are constructed entirely ofwelded walls, they have sufficient structural rigidity to permitshipping them in a horizontal position; the packages are placed on therailroad car with the long axis parallel to the length of the car.Installation at the site consists simply of swinging the assemblies intoa vertical position and setting them on four piers or other suitableload pedestals. The only pressure part welding consists of joining alimited number of feeder and release tubes, as has been described above.With this design, the diameter of the drum is no longer a factor becauseof shipping limitations. This makes it possible to use 54 and 60 inchdiameter drums thereby permitting the installation of adequate steamseparating equipment. These drum sizes are identical to the drumspresently used in field-erected boilers. Consequently, steam puritylevels can be comfortably maintained in spite of severe loadfluctuations and, certainly, with better purity than has beenexperienced with conventional package boilers.

lt has long been known that a rectangular furnace cross section is notthe ideal shape for uniform radiant heat absorption along the periphery.In the ideal case. a point source of radiation in the center ofa circlewill radiate uniformly to the circumference. FIG. 4 shows the closeapproximation to the ideal that has been achieved with the present steamgenerating unit. The fuel and air are injected through the burners in adiagonally-opposed fashion with the trajectories meeting approximatelyin the center of the octagonal furnace. The resultant high intensitycore of combustion is ,a single oblong flame mass that radiates to allwalls in a uniform manner. The high velocity jets from both sidesimpinge on each other and the momentum is converted into secondaryatomization of liquid fuels and a high degree of turbulence. In thepreferred embodiment, four burners would be associated with each of theinclined panels to give a total of eight burners and experience hasshown that the tumdown which is possible is far greater than with mostexisting boilers. Although opposed firing is maintained to retainuniformity in heat absorption and gas flow for continuous operation,under light load conditions and for the cleaning of burner guns and thelike, individual burners can be removed from operation withoutdetrimental effects. The present design provides for access forinspection, repair of cleaning of the entire generating bank,superheater, and furnace. [t is clear, then, that the present design hasprovided a shop-assembled boiler that has all the characteristics ofproven field-erected designs without excluding the economic advantagesof shop assembly.

It is obvious that minor changes may be made in the form 1 andconstruction of the invention without departing from the material spiritthereof. It is not, however, desired to confine the invention to theexact form herein shown and described, but it is desired to include allsuch as properly come within the scope claimed.

lclaim:

l. A steam generating unit, comprising:

a. a furnace section of welded wall construction of vertically-elongatedform;

b. a boiler section of vertically-elongated form mounted parallel andadjacent to the furnace section;

c. feeder tubes connecting the two sections at their lower ends;

d. steam release tubes joining the two sections at their upper ends;

e. each of said sections having cross-sectional dimensions permitting itto be accommodated with its length horizontal on a railroad flatcar withadequate overhead and side clearance;

f. each of said sections further being of such'complete and integralconstruction as to permit self support with its length horizontal and asto require for completion of its pressure parts only the connection offeeder and steam release tubes to the other section; and wherein thefurnace section is provided with a bottom consisting of a flat centralportion joined to an upwardly-inclined portion on each side and whereina header extends transversely across the said bottom with horizontal andinclined portions lying in thec'entral and inclined portions,respectively.

2. A steam generating unit as recited in claim 1, wherein the saidfeeder tubes extend rearwardly from the header and are welded tocorresponding feeder tubes extending forwardly from the boiler section.

3. A steam generating unit, comprising: a. a furnace section of weldedwall construction of vertically-elongated form;

b. a boiler section of vertically-elongated form mounted parallel andadjacent to the furnace section;

c. feeder tubes connecting the two sections at their lower ends;

d. steam release tubes joining the two sections at their upper ends;

e. each of said sections having cross-sectional dimensions permitting itto be accommodated with its length horizontal on a railroad flatcar withadequate overhead and side clearance; and

. each of said sections further being of such complete and integralconstruction as to permit self support with its length horizontal and asto require for completion of its pressure parts only the connection offeeder and steam release tubes to the other section; and wherein thefurnace section is provided with a roof consisting of a horizontalcentral portion joined to a downwardlyinclined portion on each side andwherein a header extends transversely across the said roof withhorizontal and inclined portions lying in the central and inclinedportions of the roof.

4. A steam generating unit as recited in claim 6. wherein the said steamrelease tubes extend rearwardly from the header and are welded tocorresponding steam release tubes extending forwardly from the boilersection.

5. A steam generating unit, comprising a furnace section of welded wallconstruction of vertically-elongated form, a boiler section ofvertically-elongated form mounted parallel and adjacent to the furnacesection, feeder tubes connecting the two sections at their lower ends,steam release tubes joining the two sections at their upper ends, saidfurnace section having a cross-sectional shape forming a polygon of morethan four sides arranged to approximate a circle, and burners located onat least two of said sides and positioned to produce high velocity fueland air jets which meet and impinge on each other approximately in thecenter of said cross section to produce a high intensity core ofcombustion in a single oblong flame mass that radiates to all of saidfurnace walls in a uniform manner.

6. A steam generating unit comprising a furnace section of welded wallconstruction of vertically-elongated form, a boiler section ofvertically-elongated form mounted parallel and adjacent to the furnacesection, feeder tubes connecting the two sections at their lower ends,steam release tubes joining the two sections at their upper ends, saidfurnace section having a rear wall and said boiler section having afront wall located in juxtaposition to the rear wall of the furnacesection, said boiler section having a front wall including two sidepanels lying in a transverse plane and defining a gap between them, therear wall of the furnace section having a central panel which lies insaid gap and two forwardly inclined panels which lie at a substantialhorizontal angle to the said side panels of the front wall of the boilersection.

7. A steam generating unit as recited in claim 6, wherein each sidepanel of the front wall of the boiler section and its correspondingforwardly-inclined panel of the rear wall of the furnace section form aspace of triangular horizontal crosssectional shape, wherein a burnerbox is located in the space, and wherein a plurality ofvertically-spaced burners are mounted in the burner box and extendthrough the said forwardlyinclined panel.

8. A steam generating unit as recited in claim 7, wherein the burner boxis provided with a baffle and dampers to permit delivery of airselectively to an individual burner.

9. A steam generating unit as recited in claim 6, wherein the said gapin the front wall of the boiler section is provided with widely-spacedtubes which extend from the upper drum to the lower drum and which arenot joined in their intermediate portions.

10. A steam generating unit as recited in claim 9, wherein the centralpanel of the rear wall of the furnace portion has some of its tubes bentout of the .plane of the panel in the upper portion to form a gas exitopening, and wherein the tube spacing and location in longitudinalvertical planes is the same as the spacing and location of the tubes inthe widely-spaced tubes in the said gap.

11. A steam generating unit as recited in claim 5, wherein the boilersection has an upper and a lower drum joined by a mass of downcomertubes, wherein the drums are joined by-a front, rear. and two side wallsformed of closely-spaced tubes joined by welded strips, and wherein abaffle extends upper end, wherein the furnace section is provided withrearwardly-directed feeder tubes at the lower end and steam releasetubes at the upper end. wherein the sections are independentlytransported to a construction site where they are mounted in paralleljuxtaposition and the feeder and steam release tubes are weldedtogether.

13. A steam generating unit, comprising:

a. a furnace section of welded wall construction of vertically-elongatedform, said welded wall construction comprising vertical tubesinterspersed with flanges welded thereto along their lengths thecross-sectional shape of the furnace section being polygonal andincluding a rear wall,

b. a boiler section of vertically-elongate form mounted parallel andadjacent to the furnace section, and

. burners located on two sidewalls of said furnace section which areadjacent to and angularly arranged relative to said rear wall, thefurnace section being provided with a bottom consisting of a flatcentral portion joined to an upwardly-inclined portion on each side,said upwardlyinclined portions being formed of constructions of thetubes and flanges of said sidewalls.

14. A steam generating unit as recited in claim 13, wherein a headerextends transversely across the said bottom with horizontal and inclinedportions lying in the central and inclined portions, respectively.

15. A steam generating unit as recited in claim 14, wherein the furnacesection is provided with a roof consisting of a horizontal centralportion joined to a downwardly-inclined portion on each side and whereina header extends transversely across the said roof with horizontal andinclined portions lying in the central and inclined portions of theroof.

16. A furnace structure comprising tube panel walls having paralleltubes interspersed with flanges, said walls being arranged to surroundan interior region, the ends of the tubes forming the panel walls beingbent inwardly of said enclosure toward a common line at each end of theenclosure, a header at each end of said enclosure and lying along saidcommon line, said majority of the tubes being connected to said headersthereby to provide a structurally sound enclosure with minimal headerconnection requirements, the tubes in first selected regions of saidpanel walls being configured to form first openings for the admission ofburners into said interior region and the tubes in second selectedregions of said panel walls being configured to form second openings forthe exit of exhaust gases from said interior region.

17. A furnace structure according to claim 16 wherein the bent ends ofsome of the tubes are arranged to form inclined walls at the ends of theenclosure.

1. A steam generating unit, comprising: a. a furnace section of weldedwall construction of verticallyelongated form; b. a boiler section ofvertically-elongated form mounted parallel and adjacent to the furnacesection; c. feeder tubes connecting the two sections at their lowerends; d. steam release tubes joining the two sections at their upperends; e. each of said sections having cross-sectional dimensionspermitting it to be accommodated with its length horizontal on arailroad flatcar with adequate overhead and side clearance; f. each ofsaid sectioNs further being of such complete and integral constructionas to permit self support with its length horizontal and as to requirefor completion of its pressure parts only the connection of feeder andsteam release tubes to the other section; and wherein the furnacesection is provided with a bottom consisting of a flat central portionjoined to an upwardly-inclined portion on each side and wherein a headerextends transversely across the said bottom with horizontal and inclinedportions lying in the central and inclined portions, respectively.
 2. Asteam generating unit as recited in claim 1, wherein the said feedertubes extend rearwardly from the header and are welded to correspondingfeeder tubes extending forwardly from the boiler section.
 3. A steamgenerating unit, comprising: a. a furnace section of welded wallconstruction of vertically-elongated form; b. a boiler section ofvertically-elongated form mounted parallel and adjacent to the furnacesection; c. feeder tubes connecting the two sections at their lowerends; d. steam release tubes joining the two sections at their upperends; e. each of said sections having cross-sectional dimensionspermitting it to be accommodated with its length horizontal on arailroad flatcar with adequate overhead and side clearance; and f. eachof said sections further being of such complete and integralconstruction as to permit self support with its length horizontal and asto require for completion of its pressure parts only the connection offeeder and steam release tubes to the other section; and wherein thefurnace section is provided with a roof consisting of a horizontalcentral portion joined to a downwardly-inclined portion on each side andwherein a header extends transversely across the said roof withhorizontal and inclined portions lying in the central and inclinedportions of the roof.
 4. A steam generating unit as recited in claim 6,wherein the said steam release tubes extend rearwardly from the headerand are welded to corresponding steam release tubes extending forwardlyfrom the boiler section.
 5. A steam generating unit, comprising afurnace section of welded wall construction of vertically-elongatedform, a boiler section of vertically-elongated form mounted parallel andadjacent to the furnace section, feeder tubes connecting the twosections at their lower ends, steam release tubes joining the twosections at their upper ends, said furnace section having across-sectional shape forming a polygon of more than four sides arrangedto approximate a circle, and burners located on at least two of saidsides and positioned to produce high velocity fuel and air jets whichmeet and impinge on each other approximately in the center of said crosssection to produce a high intensity core of combustion in a singleoblong flame mass that radiates to all of said furnace walls in auniform manner.
 6. A steam generating unit comprising a furnace sectionof welded wall construction of vertically-elongated form, a boilersection of vertically-elongated form mounted parallel and adjacent tothe furnace section, feeder tubes connecting the two sections at theirlower ends, steam release tubes joining the two sections at their upperends, said furnace section having a rear wall and said boiler sectionhaving a front wall located in juxtaposition to the rear wall of thefurnace section, said boiler section having a front wall including twoside panels lying in a transverse plane and defining a gap between them,the rear wall of the furnace section having a central panel which liesin said gap and two forwardly inclined panels which lie at a substantialhorizontal angle to the said side panels of the front wall of the boilersection.
 7. A steam generating unit as recited in claim 6, wherein eachside panel of the front wall of the boiler section and its correspondingforwardly-inclined panel of the rear wall of the furnace section form aspace of triangular horizontal cross-sectional shape, wherein a burnerbox is located in the space, and wherein a plurality ofvertically-spaced burners are mounted in the burner box and extendthrough the said forwardly-inclined panel.
 8. A steam generating unit asrecited in claim 7, wherein the burner box is provided with a baffle anddampers to permit delivery of air selectively to an individual burner.9. A steam generating unit as recited in claim 6, wherein the said gapin the front wall of the boiler section is provided with widely-spacedtubes which extend from the upper drum to the lower drum and which arenot joined in their intermediate portions.
 10. A steam generating unitas recited in claim 9, wherein the central panel of the rear wall of thefurnace portion has some of its tubes bent out of the plane of the panelin the upper portion to form a gas exit opening, and wherein the tubespacing and location in longitudinal vertical planes is the same as thespacing and location of the tubes in the widely-spaced tubes in the saidgap.
 11. A steam generating unit as recited in claim 5, wherein theboiler section has an upper and a lower drum joined by a mass ofdowncomer tubes, wherein the drums are joined by a front, rear, and twoside walls formed of closely-spaced tubes joined by welded strips, andwherein a baffle extends downwardly from the upper drum to divide thespace formed by the said walls into a forward pass in which asuperheater is located and a rearward pass in which the said mass ofdowncomer tubes is located.
 12. A steam generating unit as recited inclaim 6, wherein the furnace section and the boiler section areindependently manufactured at a location remote from a constructionsite, wherein the boiler section is provided with forwardly-directedfeeder tubes at the lower end and steam release tubes at the upper end,wherein the furnace section is provided with rearwardly-directed feedertubes at the lower end and steam release tubes at the upper end, whereinthe sections are independently transported to a construction site wherethey are mounted in parallel juxtaposition and the feeder and steamrelease tubes are welded together.
 13. A steam generating unit,comprising: a. a furnace section of welded wall construction ofvertically-elongated form, said welded wall construction comprisingvertical tubes interspersed with flanges welded thereto along theirlengths the cross-sectional shape of the furnace section being polygonaland including a rear wall, b. a boiler section of vertically-elongateform mounted parallel and adjacent to the furnace section, and c.burners located on two sidewalls of said furnace section which areadjacent to and angularly arranged relative to said rear wall, thefurnace section being provided with a bottom consisting of a flatcentral portion joined to an upwardly-inclined portion on each side,said upwardly-inclined portions being formed of constructions of thetubes and flanges of said sidewalls.
 14. A steam generating unit asrecited in claim 13, wherein a header extends transversely across thesaid bottom with horizontal and inclined portions lying in the centraland inclined portions, respectively.
 15. A steam generating unit asrecited in claim 14, wherein the furnace section is provided with a roofconsisting of a horizontal central portion joined to adownwardly-inclined portion on each side and wherein a header extendstransversely across the said roof with horizontal and inclined portionslying in the central and inclined portions of the roof.
 16. A furnacestructure comprising tube panel walls having parallel tubes interspersedwith flanges, said walls being arranged to surround an interior region,the ends of the tubes forming the panel walls being bent inwardly ofsaid enclosure toward a common line at each end of the enclosure, aheader at each end of said enclosure and lying along said common line,said majority of the tubes being connected to said headers thereby toprovide a structurally sound encloSure with minimal header connectionrequirements, the tubes in first selected regions of said panel wallsbeing configured to form first openings for the admission of burnersinto said interior region and the tubes in second selected regions ofsaid panel walls being configured to form second openings for the exitof exhaust gases from said interior region.
 17. A furnace structureaccording to claim 16 wherein the bent ends of some of the tubes arearranged to form inclined walls at the ends of the enclosure.